There are many different laser applications where an output having different wavelengths would be desirable. One example is in the field of ophthalmic laser surgery. At the present time, there are a number of different eye problems which can be treated using laser radiation. For example, pan-retinal photocoagulation is often performed to improve the sight of diabetics. In this procedure, portions of the retina are burned to prevent new, weak blood vessels from forming. It has been found that in an eye where the vitreous is clear, this treatment is best performed using green light or light having a wavelength in the 500 to 540 nm regime.
Unfortunately, patients with eye troubles often have other complications. For example, weak blood vessels in the eyes of diabetics often rupture so that hemorrhaging occurs and blood cells are present in the vitreous of the eye. The hemoglobin in the blood cells is highly absorptive of green laser light. Thus, in order to treat a retina were blood cells are present in the vitreous, it is necessary to use red light which will pass through the red colored blood cells.
Still other procedures are best performed using yellow light. Yellow light is desirable because it is highly absorbed in blood and has a short penetration depth behind the retina. Recently, opthalmalogists have begun using yellow laser light to treat macular degeneration.
As can be appreciated, many ophthalmologists would prefer to purchase a single laser system which could be used to treat a wide variety of problems. Accordingly, various manufacturers have developed ophthalmic laser systems which were capable of generating a number of different wavelengths. One example of such a laser system is marketed by the assignee herein under the model number 920 A/K. This laser system includes both an argon and a krypton gas ion laser. The argon ion laser generates most of its light output in the blue/green regions of the spectrum. The predominant output of the krypton laser is at 647 nm, in the red portion of the spectrum. By selectively energizing one of the two lasers, the doctor can choose the wavelength region appropriate for the selected surgical procedure.
In order to provide even further flexibility, the assignee herein markets another ophthalmic laser system (marketed under the trademark Lambda Plus) which includes both an argon laser and a tunable dye laser. In this system, the argon laser is used to generate blue/green light while the dye laser is used to generate a tunable output covering the red and yellow wavelength regimes. This latter laser system provides significant flexibility in the selection of wavelengths.
As can be appreciated, both of the laser systems described above required two lasers in order to achieve the desired wavelength selectability. Although these laser systems are commercially successful, it would be desirable to design a system which would generate a suitable range of wavelengths in a simpler and more cost effective manner.
This goal is achieved with the system of the subject invention which utilizes only a single krypton ion laser discharge tube to generate output in multiple wavelength regions. As noted above, a krypton ion laser typically operates with an output at 647 nm, in the red regime. However, krypton gas also includes significant lasing transitions at 531 nm and 568, in the green and yellow regions respectively. Unfortunately, there is strong competition between the 647 nm line and both the 531 nm and 568 lines. Competition arises when two lasing lines vie for energy at either the upper or lower lasing level. Due to this competition, it is quite difficult to utilize a single krypton laser that could generate sufficient output powers at these different lines to effectively treat a variety of eye diseases. As discussed more fully below, this problem has been overcome using a unique mirror assembly which selectively moves one of two narrow band resonator mirrors into alignment with the laser tube. One of the two mirrors is optimized to allow the laser to generate red light while the other mirror suppresses the red light and allows the laser to operate in the green and yellow wavelength regions.
The general concept of utilizing different wavelength selective mirrors to control the output of a gain medium is not new. For example, U.S. Pat. No. 3,860,888, issued Jan. 14, 1975, to Stephens, discloses a solid state Nd:YAG laser. An output coupler is mounted at one end of the gain medium. The other end of the resonator is defined by a multifaceted mirror mounted on a rotating polygon. Each facet of the mirror has a different wavelength selective coating. By rotating the mirror support, various coated mirrors can be brought into alignment with the gain medium to vary the output wavelength of the laser.
Another approach is described in U.S. Pat. No. 4,757,507, issued Jul. 12, 1988, to Wondrazek. In this Nd:YAG laser system, two mirrors having different wavelength coatings are mounted on a planar support. The output wavelength of the laser is varied by translating the support along an axis perpendicular to the optical axis of the gain medium.
A still further approach for switching laser mirrors is disclosed in U.S. Pat. No. 5,048,034, issued Sep. 10, 1991, to Tulip. In this Nd:YAG system, a pair of wavelength selective mirrors are mounted on a pulley system. The output wavelength of the laser is varied by moving the pulley system which functions to selectively align one of the mirrors with the optical axis of the gain medium.
This general concept was also described by the assignee herein for use with gas ion lasers. This description can be found in U.S. Pat. No. 5,124,998, issued Jun. 23, 1992 to Arrigoni. In the laser system described in this patent, the gain medium is defined by a gas discharge tube. One end of the tube is sealed with a transmissive window. An output coupler is located beyond that window. The other end of the tube is sealed with the high reflector of the resonator. The high reflector is mounted at the end of a flexible bellows. The bellows is provided so that the alignment of the high reflector can be varied from the outside of the sealed enclosure.
In one of the embodiments described in U.S. Pat. No. 5,124,998, (and illustrated in FIGS. 4 and 5), a mirror is shown which is divided into two segments. Each segment is provided with a different wavelength selective coating. Using a set of adjustment screws, the end of the bellows could be laterally translated to bring one of the two segments of the mirror into alignment with the resonator. The output wavelength of the laser was dependent upon which of the two segments was aligned.
U.S. Pat. No. 5,123,998, does not contain any disclosure about the incorporation of such a device in an ophthalmic laser having a krypton gas discharge tube to permit selection of either red or yellow/green output wavelengths. In addition, it has been found that the mechanism shown in the '998 patent has certain drawbacks. More specifically, in the mechanism of the '998 patent, the free end of the bellows is forced to move laterally. This motion creates a significant transverse loading on the bellows. This transverse loading provides a resistance which makes adjustment and stabilization of the segmented end mirror quite difficult. In addition, the transverse loading places a torque on the stem of the discharge tube, adversely affecting alignment. Finally, it is quite expensive to fabricate a single mirror with two regions having different reflectivities.
Therefore, it is an object of the subject invention to provide an improved assembly for selectively moving one of two narrow band mirrors into alignment with an optical resonator.
It is a further object of the subject invention to provide a mirror assembly for connection to a bellows of gas discharge tube capable of selectively moving one of two narrow band mirrors into alignment with the tube.
It is another object of the subject invention to provide an assembly capable of selectively moving one of two optical elements into alignment with an optical axis.
It is a further object of the subject invention to provide a medical laser system having a krypton laser which can be selectively controlled to generate either red or yellow/green output wavelengths.
It is still another object of the subject invention to provide a medical laser system which is simpler in construction than prior art systems, yet provides the ability to selectively switch between different disparate output wavelengths.
It is still a further object of the subject invention to provide a medical laser system which allows the physician to rapidly switch the output between different wavelengths.